NMR breast imaging, for example, can be performed with whole body scanners by positioning the region of interest on the chest wall near the center of the imaging region. In traditional scanners, the imaging region is usually defined as a quasi-spherical volume, concentric with the center of the scanner, where the field satisfies the uniformity levels dictated by the diagnostic requirements. If such a scanner is used for breast imaging, the off-center position of the body results in a very inefficient use of a scanning instrument whose cavity has been designed to be large enough to accommodate the whole body of a patient.
In principle, any magnet can be designed to generate a field whose center is arbitrarily chosen within its cavity. Usually a shift of the center of the field away from the geometrical center of the cavity results in a lower figure of merit, i.e. more energy is required to generate the same field. If the magnet cavity were closed, the shifting of the field center would not necessarily affect the uniformity of the field. In a practical situation, however, if the cavity must be open, the figure of merit and the uniformity of the field both deteriorate rapidly as the field center approaches the magnet opening. In order to maintain the same value of the field and the same dimension of the imaging region, the size of the magnet must consequently be increased.
An example of this phenomena is a flat bed superconductive magnet where the coil system is below a flat platform 3.9 meters in diameter and the imaging region is above the platform at about 15 cm from its surface. The stray field of such a magnet is quite large, with the 5 Gauss line at more than 10 meters from the center of the structure when the field within the imaging region is 0.2 T (See "New Magnet Designs for MR", D. Hawksworth. Magnetic Resonance in Medicine, Vol. 9, pp 27-32, 1991)